1. Field of the Invention
This invention relates to a carrier regeneration circuit for a quadrature demodulator and to a method of carrier regeneration.
2. Description of the Related Art
An exemplary conventional quadrature demodulator (referred to herein at times as a four-phase demodulator) is shown in FIG. 1. Referring to FIG. 1, the quadrature demodulator includes quadrature detector 1 for performing coherent detection, multi-level code discriminator 2 for discriminating two pairs of output signals of quadrature detector 1, phase deviation detection circuit 3 for logically processing the output of multi-level code discriminator 2 and controlling voltage control oscillator 5, low-pass filter 4 for filtering unnecessary spectral components of a control signal outputted from phase deviation detection circuit 3, and voltage control oscillator 5 for regenerating a carrier with which coherent detection is to be performed at quadrature detector 1.
Based on the conventional construction, a quadrature modulated wave p is detected through coherent detection with regenerated carrier u outputted from voltage control oscillator 5 by phase detector 1 and is outputted as base band signal q. Base band signal q is inputted to multi-level code discriminator 2, from which demodulated main signals D.sub.P and D.sub.Q and deviation signals E.sub.P and E.sub.Q are outputted and inputted to phase deviation detection circuit 3. Phase deviation detection circuit 3 logically processes the signals inputted thereto and outputs control signal r for controlling voltage control oscillator 5. Control signal r passes through low-pass filter 4, which is provided for removing unnecessary waves, to make control signal s, which controls voltage control oscillator 5.
A carrier regeneration circuit for a quadrature demodulator is disclosed in JP Patent Publication No. 79904/1991 dated Dec. 20, 1991. The invention provides a carrier regeneration circuit wherein two-phase main signals obtained by quadrature amplitude modulation are detected through quadrature phase detection with the outputs of a voltage control oscillator (VCO) to make two base band signals in the form of digital signal consisting of predetermined number of bits, and the phase difference between the received carrier signal and a standard carrier signal outputted from the voltage control oscillator are correlated in accordance with a value obtained through the exclusive OR gate between the most significant digit (MSD) of one of the main signals and the least significant digit (LSD) of the other main signal, whereby the output frequency of the voltage control oscillator is controlled.
In the conventional carrier regeneration circuit employed in the quadrature demodulator described above, while phase deviation information is extracted and controlled to bring the phase deviation to zero, an AFC (auto frequency control) control signal cannot be extracted, and it is very difficult to eliminate false pull-in of a regenerated carrier, that is, the phenomenon that the frequency of the voltage control oscillator is stabilized at a frequency different from its correct lock frequency.